The invention relates to a cold cathode for discharge lamps, in particular for discharge lamps which are discharge.
In dielectrically impeded discharges, at least one electrode is separated from the discharge space by a dielectric layer.
The invention also relates to a discharge lamp having this cold cathode, in particular to a discharge lamp which is operated by means of a dielectrically impeded discharge, as well as to a method for operating such lamps.
Here, the term "discharge lamp" signifies radiators which emit light, that is to say visible electromagnetic radiation, or else ultraviolet (UV) and vacuum ultraviolet (VUV) radiation.
The task of the electrodes of discharge lamps is, inter alia, to supply the number of free electrons necessary for maintaining a self-maintained discharge. These electrons are essentially supplied by the cathode (in the case of operation with a voltage of invariable polarity, for example DC voltage or unipolar pulsed voltage) or by the instantaneous cathode (in the case of operation with a voltage of variable polarity, for example AC voltage or bipolar pulsed voltage). Furthermore, free electrons are generated in the cathode fall space positioned in front of the (instantaneous) cathode.
The cathode fall space is characterized by a field strength which is high by comparison with the remaining space between the electrodes, that is to say the electric potential with respect to the cathode drops relatively strongly in this region. Consequently, a generation of free electrons in the cathode fall space is linked in general to a correspondingly high power turnover. With regard to as efficient as possible a generation of useful radiation, that is to say of light or UV/VUV radiation, the aim is to reduce the cathode fall of discharge lamps. In the final analysis, this requires increasing the efficiency with which electrons emerge from the cathode surface.
One possibility consists in designing the electrodes as filaments, heating them and, if appropriate, additionally coating them with an emitter paste, in order thereby to improve the thermal electron emission. This relatively complicated technique is applied, for example, in the case of fluorescent lamps. Further disadvantages associated with these so-called hot electrodes are the limited service life of the lamp and the heating up of the filling gas.
DE-U 295 01 343 discloses a glow fluorescent lamp having a cold cathode. The electrodes are doped with manganese or lanthanum in order to reduce the cathode fall.
U.S. Pat. No. 5,418,424 describes a VUV radiation source having a photocathode. Formed inside the radiation source, inter alia, are Xe excimers which generate shortwave VUV radiation. The photocathode comprises a 50 nm thick photoemitting layer, applied to a stainless steel surface, and a grid electrode in parallel with said layer. The VUV radiation passes through the grid electrode onto the photoemitting layer and releases electrons there by means of the photo effect. These electrons then pass through the grid electrode and thus maintain the discharge. A disadvantage is that the described mechanism for generating electrons is based on the VUV radiation generated by the discharge itself. Thus, during ignition of the discharge, this electron source is not yet available, or is so only to a limited extent. This can be seen in a negative fashion in a loss of efficiency in the case of discharges operated in pulses.
EP 675 520 A2 discloses a pot-shaped or beaker-shaped electrode for miniature neon lamps for automobile illumination. The inner wall of the electrode is uniformly covered with an emitter layer.
Finally, U.S. Pat. No. 5,159,238 describes a cathode for flat discharge lamps. The cathode comprises electrically conductive oxide particles which are embedded in a low-melting glass, for example lead glass.